Reference is made to co-pending application Ser. No. 08/569,729, filed Dec. 8, 1995, entitled "Hydraulic Tensioner With Check Valve Vent," which is assigned to the assignee of the present application and which is incorporated herein by reference.
The present invention relates to a hydraulic tensioner having a housing which includes a fluid chamber and a hollow piston slidably received within the chamber. More particularly, the present invention relates to such a tensioner with a sealing element positioned between the piston and the bore or inner lateral wall of the chamber to provide control of fluid leakage from the chamber.
Tensioning devices, such as hydraulic tensioners, are used as a control device for automotive timing drives. An automotive timing drive includes either a chain or belt which travels between a plurality of sprockets or pulleys. In an automotive application, the tension of the chain can vary greatly on the slack side of the chain as the chain travels between the sprockets. Likewise, the tension of the chain can further vary as the chain stretches due to wear. As a result, it is important to impart and maintain a certain degree of tension to the chain to maintain control over the chain as it travels between the sprockets. As the chain tension lessens due to wear, it is important to provide an adjustable tensioning member to maintain the proper tension of the chain.
Hydraulic tensioners are well known in the art such as Hunter et al., U.S. Pat. No. 5,346,436, which is owned by the assignee of the, present application and in incorporated herein by reference. Hunter et al. discloses a hydraulic tensioner with an air vent. However, hydraulic tensioners typically do not include a sealing element or they include a sealing element that provides a fluid tight chamber.
One example of a hydraulic tensioner with a sealing element is described in Sosson, U.S. Pat. No. 4,850,941. Sosson discloses a hydraulic tensioner, which includes a piston with a groove on its outer surface for receiving a sealing element between the groove and the lateral wall of the chamber. The sealing element has lips adapted to provide a seal between the groove, i.e., the outer surface of the piston, and the lateral wall of the chamber. The chamber becomes closed and sealed owing to the presence of the sealing element. The chamber is therefore fluid tight, and leakage from the chamber is prevented.
Another example of a hydraulic tensioner, which includes a sealing element is described in Wahl, U.S. Pat. No. 4,190,025. Wahl discloses a hydraulic tensioner, which includes an annular oil retainer piston with circumferential grooves for receiving inner and outer O-ring seals for sealingly engaging a plunger and the lateral wall of the chamber. The O-ring seals of Wahl provide a fluid tight chamber, and therefore leakage from the chamber is prevented.
The sealing element of the present invention, however, is a simple and inexpensive apparatus constructed to permit controlled fluid leakage from the chamber. Fluid flow out of the chamber is dependent on certain factors including the outer diameter of the piston, the inner diameter of the chamber, and the clearance between the outer diameter of the piston and the inner diameter of the chamber. In positioning a sealing element between the inner wall of the chamber and the outer wall of the piston, fluid flow out of the chamber can be limited and controlled. In particular, fluid flow out of the chamber can be limited and controlled by regulation of the size of a discontinuity, i.e., a space or gap, in the sealing element.